Flash flood warning operations during tropical rainfall in low relief terrain

Flash flood forecasting capabilities in the NWS have undergone significant changes with the implementation of the Flash Flood Monitoring and Prediction (FFMP) program in 2002. FFMP is a suite of software used by Weather Forecast Office (WFO) forecasters to determine if small basins or areas within a County Warning Area (CWA) have accumulated rainfall in excess of Flash Flood Guidance (provided by the servicing River Forecast Center (RFC)). FFMP calculates rainfall accumulations on time scales ranging from ½ hour to 6 hours for each small basin. These small basins, represented as vector lists in files located on AWIPS for use by FFMP, were originally developed by the National Severe Storms Laboratory (NSSL) National Basin Delineation Project using the USGS National Elevation Dataset (NED). Basin sizes were determined such that most basins within each radar umbrella, were assigned at least one 256 level, 1km x 1degree Digital Hybrid Reflectivity (DHR) radar bin.

This paper examines the flash flood that occurred over Southwest Georgia, as a result of heavy rainfall from a feeder band associated with Hurricane Dennis after Dennis moved into Southwest Alabama the evening of 10 July. The analysis scale of various precipitation products (including FFMP), available to the Weather Forecast Office (WFO) forecaster, and the timing and hydrology of the resultant floods are reviewed. Staff warning decisions are also discussed. Field verification of the flash flood and feedback from Emergency Management officials is presented.